The invention relates to a method for producing hot strip from magnesium wrought alloys. Magnesium is the metal with the lowest density, has strength characteristics similar to those of aluminium, and could substitute for this as a lightweight construction material. An important precondition for the progress of magnesium as a lightweight construction material, however, is the availability of economically-produced sheet materials. Magnesium sheets are at the present time only available on the market in small quantities and at high prices. This is explained by the substantial effort and expense which is required in hot-rolling sheets or strip of magnesium wrought alloys according to the present state of the art. This is described in detail in the Magnesium Taschenbuch (Aluminium-Verlag Düsseldorf, 2000, 1st edition, pp. 425 to 429). One basic problem with the hot-rolling of sheets of Mg wrought alloys lies in the fact that the conventional raw material from ingot casting or continuous casting solidifies in large grain and porous form, as well as containing pronounced segregations and coarse precipitations. The cast ingots are in many cases subjected to a homogenisation annealing process, and then hot-rolled at temperatures of between approx. 200 and 450° C. These procedures in most cases require in part repeated intermediate heating of the rolling stock, since otherwise wastage is incurred due to crack formation.
Attempts have been made to improve the deformability and the properties of a hot-rolled magnesium strip by the production of suitable raw materials, from which the hot strip is then rolled. Such a method is known, for example, from U.S. Pat. No. 5,316,598. According to the known method, magnesium powder compressed at temperatures from 150-275° C. solidifies rapidly. By extruding or forging, a raw material is produced from this ingot which is then rolled to form a sheet with a thickness of at least 0.5 mm. The rolling temperatures in this situation lie at between 200° C. and 300° C. The magnesium hot strip which is obtained in this way exhibits superplastic properties and at room temperature has high strength and good toughness in the rolling direction.
A disadvantage with the known method, however, is that for the manufacture of the raw material a magnesium powder is first produced, this powder is compressed, and an accelerated cooling process must then be carried out. The effort and expenditure in terms of apparatus and personnel associated with this leads to high manufacturing costs. In addition to this, it has been shown that the deformation of the raw material in the course of hot-rolling is difficult to master despite the elaborate production of the raw material.
In addition to the aforementioned state of the art, a method is known from JP 06293944 A for the manufacture of a magnesium sheet, in which a slab is first cast from a melt containing 0.5-1.5% REM, 0.1-0.6% zirconium, 2.0-4.0% zinc, and magnesium as the remainder. This slab is then hot-rolled in two stages, whereby in the second stage of the hot-rolling the rolling temperatures lie between 180-230° C., for preference 180-200° C., and a total deformation is achieved of 40-70%, for preference 40-60%. The strip obtained in this way is said to possess good deformability. The hot-rolling carried out in two stages, however, also makes the rolling process, and the temperature controlling which is to be maintained, elaborate and expensive and difficult to master.